Wich rendering engine is the best?
They are different engines - for different purposes.

LuxCoreRender is an extremely advanced, physically accurate engine. It supports both path tracing (rays are shot from the camera) and bidirectional path tracing (rays are shot from both the camera and light sources). LuxCoreRender also has multiple sampling types, along with many other state-of-the-art features that are hard to find even in professional paid render engines. There are also many quality-of-life features that LuxCoreRender has, that make it fast for certain tasks.
 LuxCore does have disadvantages, however. The biggest is it's absolutely terrible motion blur. Because it's motion blur is so bad, it should not be used for animation, like at all. 

Cycles is very good as well. It's more typical, and well rounded. You know cycles, it's good! It struggles with things like caustics and dispersion, though. 

Here's the final verdict from me. Cycles works pretty great most of the time. And, I recommend using it most of the time. It's shader system is excellent, and allows for incredible effects. LuxCoreRender is amazing too, but is only worth using for archviz and scenes with lots of glass and complex stuff like that. LuxCoreRender was actually developed primarily for archviz, by the way. 

I hope this helps!

EDIT: about biases; cycles is partially biased, and LuxCore can be completely unbiased, but can also be biased depending on what you want for that specific scene. In LuxCore, you can control it